There are a few options available, depending on what your needs are exactly.
Option 1 - Manually route specific calls through google voice
Before I had a smartphone, this was how I routed all my calls. Basically, for every contact, I stored their regular number as well as a Google Voice number. The GV number was my own GV number, followed by a
wait, then a
wait, and finally followed by the contact's number and a pound key. In effect, I made a macro.
Ex ( My #: 1234567890, Bob's number: 19876543210):
If you only have a few contacts that you wish to call via your Google Voice number, this option is very simple. It does add some extra time to call them, as your phone doesn't connect directly to your contact, but rather connects to Google Voice, then "manually" dials the contact's number.
Note that you must have certain settings set up in Google Voice (the website, not the app) so that when you call your own number, you get the Google Voice menu system instead of leaving yourself a message. Also, if you require a PIN for your specific phone, you'll have to modify the setting, or add a PIN to the macro dial string.
FYI, this was how I got unlimited (outgoing) minutes as well. Since I could choose up 5 numbers to dial without using minutes, I simply chose my own Google Voice number as one of the five. Since all my calls went through GV, my carrier simply saw me dialing the same number all the time.
Option 2 - Add a widget to simplify Google Voice
The Google Voice app has several widgets, and one of them is a 1x1 icon that rotates between the 4 calling options:
Always use Google Voice,
Only for International Calls,
Ask Everytime, and
Never use Google Voice.
This is the option I used to prefer, and I have actually used this in the past. It is very useful to quickly switch phone numbers, and you don't need to go through the hassle of opening the Google Voice app and finding the specific setting. Just click the widget until it is on the setting you want, and call from there.
Option 3 - Use Titanium Backup Pro to make a separate profile
With the paid version of Titanium Backup, you can create multiple profiles for apps, essentially giving you the ability to have multiple users. This is the option I use currently, as it reduces the number of clicks I need, and it allows me to have multiple profiles on a specific set of apps I want (such as games, lists, and Google Voice). I can switch from my Normal profile to my Guest profile (for example) so that I can hand off my phone to a family member to play games without messing up my own progress.
Set up a second profile that has Google Voice on for all calls, and make your primary profile never ask. When you want to use google voice, just switch profiles.
There are other apps that let you do multiple profiles, and I know some of them are free if you only want a total of 2 profiles.